The present invention relates generally to electronic musical instruments. More particularly, the invention relates to a signal processing device for altering the spectral content and sustain characteristics of electronic musical instruments such as electric guitars and the like.
The popularity of electric guitars and other electronic musical instruments has created quite a demand for signal processing devices to change the timber, overtones, coloration, sustain and other musical properties of the instrument. Today electronic musical instruments are played through amplifiers, audio mixing consoles and even portable headphone amplifiers. Regardless of the final output device, be it amplified speakers, mixing console or headphones, many musicians prefer using standalone signal processing devices to enhance their sound.
One type of signal processing device is the distortion booster which produces a fuzz-like tone by overdriving cascaded amplifier stages. There are many popular overdrive units on the market, many of them being transistorized battery-powered circuits housed in small foot pedal boxes. Occasionally, these transistorized distortion pedals have a jack for connection to an external "battery eliminator" power supply. These power supplies typically comprise a transformer and rectifier circuit in a self-contained module intended to be plugged directly into household AC current. Battery eliminators of this type typically supply either 6 or 9 volt DC output.
When several amplifier stages are cascaded together and biased to overdrive, nonlinearity results. Transistor circuits are used in foot pedal distortion booster circuits because they are economical and can be operated on low DC voltages. When driven into the nonlinear regime, transistor circuits tend to produce tonal harmonics which are odd multiples of the original or fundamental tone. This is what produces the distortion or fuzz-like sound. For some types of music, such sound is considered by some to be desirable.
However, many musicians find that the addition of odd harmonics to the fundamental tone lacks musical aesthetic qualities. It has been found that the even harmonics are more pleasing to many and would be preferred over the odd harmonics produced by transistor circuitry. Since naturally occurring musical tones and the scales and chords upon which modern music are based all employ even harmonics, it is advantageous to accentuate these harmonics by signal processing.
Although virtually replaced by transistor and semiconductor devices, vacuum tubes naturally provide the beneficial quality of generating even harmonics when overdriven to the point of distortion. There are some musical instrument amplifiers on the market today which use vacuum tube circuitry to gain this advantage. However, because of packaging and power supply constraints, there has not heretofore been a stand-alone distortion pedal operable by battery eliminator DC power supplies which use vacuum tubes. Indeed, the classical vacuum tube bias circuit often employs voltages in the range of ninety to several hundred volts for the plate supply, with 6 volt and 12 volt filament heater voltages being common. Thus traditional vacuum tube circuits simply are not designed to work with a 9 volt battery eliminator power supply of the type commonly used with transistorized distortion pedals. Of the very few distortion pedals which use vacuum tubes, all have 110 volt AC power cords like lamps, toasters and other household appliances.
Several problems arise when AC power is used in this conventional fashion. First, distortion pedals employing conventional AC power cords require step down transformers and rectification circuitry to be built into the foot pedal housing. Placement of the transformer in close proximity to the vacuum tube creates a significant problem with electromagnetically induced hum, an extremely undesirable quality in the musical instrument industry. In addition, because these devices operate at full line current, the components are more expensive and require Underwriter's Laboratories' approval. As a result, AC powered distortion pedals tend to cost approximately twice what the transistorized pedal costs.
Furthermore, the requirement of needing a separate AC outlet to operate a distortion pedal can be a significant drawback to the studio musician or performing musician. In the studio and on stage, there is often a shortage of AC outlets and also often quite a tangle of cables and wires leading between the instruments, microphones, amplifiers, mixing consoles and the like. The proximity of AC power cords to high impedance, high gain circuitry found in this equipment can induce unwanted hum. Also the AC power cord of a distortion pedal can create ground loop problems as well.
The present invention provides a unique and novel vacuum tube overdrive distortion pedal which is capable of operating from a low voltage "battery eliminator" DC power supply. The presently preferred embodiment employs a 12 AX7A vacuum tube (filament heater requirement 12 volts) and a 9 volt DC battery eliminator serves as the power supply. The invention provides a rich, warm distortion emphasizing even harmonics. The invention thus rivals the sound qualities of the finest AC powered tube distortion circuits, without any of the power supply problems described above.
The invention is preferably packaged in a small foot pedal with jack on the rear into which the conventional battery eliminator low voltage conductors are plugged. The foot pedal conforms to the size, shape and power requirements of popular transistorized effects devices. Thus the invention can be connected together with other effects devices to a common battery eliminator supply. There are effects pedal carrying cases on the market today which accommodate several interconnected effects pedals and which supply the pedals with the proper DC operating voltage. The distortion pedal of the invention will operate with such carrying cases, making it popular with traveling musicians.
The signal processing device of the invention employs a unique biasing circuit with pull-up circuitry which references the vacuum tube grid to the B+ supply voltage, rather than to ground as is the conventional practice. This gives the circuit a desirable high input impedance, increases nondistorted headroom substantially and lowers output impedance of the device. Although not fully understood, it is believed that the circuit also produces a secondary emission from the grid by pulling it slightly positive, allowing electrons from the cathode to move more freely toward the plate. Notably, the power supply voltage is also lower than the conventional requirements for filament heater operation.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.